A man alleged to have been part of a swingers club which sexually abused a string of children in Perth’s northern suburbs has pleaded guilty to four counts of abusing his stepdaughter.

The 45-year-old man, who appeared in Perth Magistrates Court this morning, admitted that three days after Christmas in 2014 he sexually penetrated and encouraged the young girl to indulge in sex with him at an address in Mullalloo.

The girl, who was his stepdaughter, was about five when the abuse began in 2011, WA police have previously revealed.

The man, who is facing another 75 charges relating to abuse of his own children, another stepchild, and a 14-year-old girl not related to him, will be back in court in June and July – when a sentencing date will be set.

WA Police allege the man, his wife and a friend, all took part in sustained and horrific abuse of various children, some of whom were just four when their ordeals began.

The mother, 39, also appeared in court today and had her raft of charges put off until May. Her lawyer told the court that 24 of those charges had been amended to change the alleged victim.

The woman, who was originally granted bail, was remanded in custody by magistrate Diane Scaddan, who refused to let her talk when she asked to be allowed to speak.

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The investigation sent shockwaves through the state last month when the stepfather, his wife and a friend were all charged in relation to alleged sex offences against her daughter.

All three have had charges added since the investigation was first revealed.

The stepfather is now charged with 79 offences, including two counts of having a sexual relationship with a child and two counts of indecently dealing with a child relating to his biological daughters, aged four at the time of the first alleged offence.

They are said to have taken place between 2001 and 2006.

He also faces 65 charges relating to his stepdaughter and stepson - aged five and six at the time of the first alleged offence.

Among those allegations are charges relating to using drugs to carry out sexual acts, and rape between 2011 and 2015.

He also faces a further 10 charges relating to a 14-year-old girl. Offences alleged to have taken place in 2011.

A 56-year-old man, a friend of the couple, faces 22 charges relating to alleged sexual assaults on a boy and a girl, aged five and six at the time of the first offence, between 2011 and 2015.

Their arrests are part of the ongoing Taskforce Mirzam investigation in WA into child sex offences that saw the arrest of a Perth mum - accused of raping her daughter.

Detectives are still investigating allegations the woman took her child to swingers’ parties and let several adult guests sexually abuse her between 2011 and 2016.

The third accused, the friend of the couple, will appear in court in May.